Marine Nuisance Species

Taxonomic Classification

Phylum Chordata, Subphylum Tunicata, Class Ascidiacea, Order Aplousobranchia, Family Didemnidae, Genus Didemnum.

Summary of Occurrence

Within the past few years, several species of the colonial tunicate genus Didemnum from different parts of the world have drawn attention as nuisance species because they reproduce rapidly and foul marine habitats (including shellfish aquacultures and fishing grounds), ship's hulls, and maritime structures. Two species have been formally named: Didemnum vexillum described from the North and South Islands of New Zealand (Kott, 2002); and Didemnum vestum, described from Portsmouth Harbor, New Hampshire, USA (Kott, 2004). Species of the genus Didemnum that are possibly different but related to D. vexillum and D. vestum have been recorded from Europe, the U.S. west coast, British Columbia, and from coastal and offshore regions of New England other than New Hampshire, and recently from Japan. These organisms have not yet been formally compared to either D. vexillum or D. vestum in the published literature, and here they are provisionally called Didemnum sp. for purposes of communication.

The rapid spread of Didemnum colonies alters marine habitats and threatens to interfere with fishing, aquaculture, and other coastal and offshore activities. The colonies shown here are found on hard substrates that include dock structures and floats, wood and metal pilings, moorings and ropes, steel chain, automobile tires, polythene plastic, rock outcrops, gravel seabed (pebbles, cobbles, boulders), and ship hulls. They overgrow organisms such as tunicates, sponges, macroalgae, hydroids, anemones, bryozoans, scallops, mussels, and oysters. Where these colonies occur on the seabed, they likely cover the siphons of infaunal bivalves. The colonies have been found at water depths ranging from intertidal to continental shelf depths of 65m (213 ft).

Gross Morphology and Growth Habits

Didemnum colonies shown here exhibit a wide variety of morphological variants that range from: 1) long, ropey or beard-like colonies that commonly hang from hard substrates such as docks, lines, and ship hulls; to 2) low, undulating mats with short surficial appendages that encrust and drape rocky seabeds (pebbles, cobbles, boulders, and rock outcrops).

Purpose and attribution

The goal of this website is to assemble and communicate information on the distribution, biology, and marine habitat impacts of the nuisance species of Didemnum. Researchers and others are encouraged to share their observations on these topics.

Contributions to the website can be sent to Page Valentine, USGS, pvalentine@usgs.gov

Website design: Donna Newman, USGS

Image processing and archival: Dann Blackwood, USGS